GIMP is the most popular, and arguably the best free photo editing software out there. Not only is GIMP fully customizable, but it's also incredibly easy to use with a simple, powerful interface. Due to its extreme popularity, there is a myriad of tutorials out there, and also lots of user-created scripts, brushes, plugins and more that are free for anyone to use—the blessing of open source software. Frankly, little would be missed from a switch from Photoshop to GIMP—this free, open-source software is getting better all the time with constant updates and compatibility with virtually everything, from Photoshop plugins to RAW format and more.